Engine wants to stall at idle and engine is warmed up. Rough idle causes a slight rattle sound due to vibration.
you may have a vacuum leak. you may have a cylinder that is misfiring. you may have a bad idle air control valve. is the check engine light on? rough idle most of the time on those is the motor mount on the passenger fender i have put a ton of them on.
Search Ford Focus Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford Focus Experts